The inverter-driven device includes a motor, an uninterruptible power supply (UPS), an electromagnetic cooker, and lighting or the like. Any device may suffer insulation degradation due to aging degradation. For example, in case of a motor used for a transport machine, movement of a platform coupled to the motor may cause friction, twist or extension and contraction in a power feeding conductor cable, resulting in a damaged conductor film. In another case of a motor used for a cutting machine, a cutting fluid, oil or the like may be scattered onto the motor, and then run through a shaft thereof to encroach insulating materials inside the motor.
In such a way, a degree of insulation degradation can be varied depending on an environment in use or durability of members. Once a leakage current flows through a portion where this insulation degradation occurs, causing a risk of electrifying human bodies or working an earth leakage breaker. The earth leakage breaker is installed for preventing the electrification of the human body. As a matter of course, the first priority is given to human life. However, once the earth leakage breaker is operated, apparatuses or facilities including the load device of interest are forced to be stopped. Thus, it will take a lot of time to identify how and where the electric leakage has been caused as well as to restore the apparatuses or facilities, thereby lowering the operation efficiency thereof.
Therefore, it is desirable from the viewpoint of condition monitoring and maintenance that an insulating state of the load device can be regularly detected and monitored. For example, as a method of detecting insulation degradation of a motor, there is proposed an insulation monitoring device for detecting insulation degradation by changing over from power-feed lines connected with a load device (e.g., motor) to a closed circuit including an insulation resistance and a ground using a switch, and then applying a voltage to the closed circuit to measure a minute electric current flowing through the closed circuit (See Patent Document 1 and others).